Among known lock devices, there is one device which is so constructed that the locking bolt is only actuated by turning the lock cylinders or thumbturn to releasably interlock with the strike. Such a lock device is conventionally attached to the door by means of mounting screws. However, as the mounting screws are usually exposed on the outer surface of the housing of the lock, the lock may be easily dismounted by an unauthorized person who has entered the house through an entrance other than the door or by using a proper key. The unauthorized person may easily unscrew the mounting screws to dismount the lock and open the door, he can thus remove a large volume of valuable objects through the door.
In view of the above disadvantage, one object of this invention is to provide a lock device of the above mentioned type in which all the mounting screws are concealed within the housing of the lock device to prevent the latter from being dismounted by a person who has entered into the room unauthorized not through the door which is locked from the outside.
Another object of this invention is to provide a lock device with two lock cylinders and one thumbturn in which the inside lock cylinder is used to releasably lock the thumbturn instead of serving as a tool for actuating the locking bolt which is actuated by turning the outside lock cylinder or the thumbturn.